With the progress of the IC (integrated circuits) industry into the ULSI (ultra large scale integration) stage, the feature size on the semiconductor chip becomes much smaller. The devices on the circuits are narrowed and densely arranged for downsizing the volume and weight of the complicate circuits for various kinds of applications. The operational speed of the circuits is raised at the same time with the smaller and faster devices.
With more and more applications of semiconductors chips on portable devices like cellular phones and laptop computers, the power consumption of the circuits must be reduced. Thus the system can operate with longer time under limited battery capacity. Besides, all the circuits on the system like computers and all kinds of controlling module on electronics are specified to operate at lower power with lower heat generation under high speed. The operating voltage level of the circuits start to decrease for providing more economical operation with low heat generation. For example, lots of circuits or logic circuits used to operate at a voltage of about 5 volts are now operates with a lower voltage, like 3.3 volts.
However, the scaling down of operating voltage cause the interfacing problems between circuits. Since not all the circuits are operated at the same low voltage level, the various circuits or chips of a system may operate at different voltage levels for various functional applications. The system must be operated by communicating signals between all the functional circuits. The input and output signals are not allowed to transfer directly without shifting in the reference voltage level for each circuit. A low voltage operated circuit can be damaged by driving with a high voltage signal. A high voltage operated circuits would operate improperly if it is driven by a low voltage signal. Thus an interface or an interface circuit must be provided for transforming between the input and output signals with distinct voltage levels.
Without limiting the application of the present invention, interface circuits for a 3.3 volts circuit and for a 5 volts circuits are taken for illustration. Turning to FIG. 1, a resistor R1 is used between a pad 10 of a first circuit 12 and a pad 14 of a second circuit 16. In the example, the first circuit 12 is operated at 3.3 volts and the second circuit 16 is operated at 5 volts. The resistor R1 is used to reduce the voltage level of the output signal from the second circuit 16. By selecting an appropriate resistance of R1, a voltage division can be achieved to provide the node 12a a voltage level of about 3.3 V, namely the operational level of the first circuit 12, when the signal from the second circuit 16 is at a high state. Thus the off state in the transistor M1 can be maintained with reduced voltage at node 12a under high voltage input.
However, additional resistor R1 are needed for the interface between each pair of the I/O (input and output) pins. The number of components needed can be great for a system with hundreds of connecting pins between the circuits. Additional components are needed and the manufacture of the circuits or the system is further complicated. Moreover, the addition of the resistors forms additional loads to each pin connection. The exchanging rate of the signals is significantly damaged since every change in the signal must drive the additional resistor before reaching another circuit. The selection on the resistance of the resistor is also very challenging. The loading condition of each pin must be considered. The speed is decreased and the reliability of the system is influenced.
Referring to FIG. 2, another prior art interface device is illustrated. Between the connection of the pad 18 of the first circuit 20 and the pad 22 of the second circuit 24, a resistor R2 is added for pulling up the voltage of node 26. However, the same problem still exist in adding an extra component of resistor R2. The signal exchanging rate is also quite limited by driving time needed through the resistor R2. The design further consumes more power by providing the resistor R2 with a DC power.